Our present objectives stem from two sets of evidence that emanate from this laboratory. Firstly, we have recently discovered that the chemical carcinogens in eight carcinogen-organ systems react principally with the same target protein, or with members of a molecular size family of target proteins in the cytosols of organs undergoing carcinogenesis. The systems involve five types of carcinogens, four different organs and three species. The findings suggest that the major target proteins in these systems are either identical, or are of a family of proteins of similar properties and functions. Secondly, in regard to the principal carcinogen-protein complex in one of these eight systems, i.e., the principal target protein of the aminoazo dyes in rat liver, we have accumulated a body of evidence which supports a model of a receptor protein capable of transporting activated carcinogen from cytoplasm to nucleus. Accordingly, we will undertake three investigative approaches concerning the existence of the principal carcinogen-protein complexes in a number of carcinogen-organ systems undergoing chemical carcinogenesis. The approaches deal with: (1) the characterization of the individual principal carcinogen-protein complexes of common molecular size in various carcinogen-organ systems, and the role of these complexes in the carcinogenic process, (2) the study of the presence and actions of activated carcinogens in certain principal carcinogen-protein complexes, and (3) the characterization of the principal carcinogen-protein complex in rat liver during carcinogenesis by N-2-fluorenylacetamide, and the role of the complex in the oncogenic process.